Multi-line lasers are commonly used in a variety of instruments including those used to optically detect and size particles down to ˜0.1 micron diameter. These instruments measure the light scattered from particles as they pass through the laser beam. Determination of the size of the particle is determined by the amount of scattered light that is detected. The noise floor in these instruments comes from light scattered by the background molecular gas in the particle-laser interaction region (background scattering noise) and is of two types: a fundamental noise from the photon statistics (shot noise) present even in a perfect laser and technical background scattering noise from an imperfect laser source (technical background scattering). The background scattering noise reduces the sensitivity of the instrument. Shot noise is not reducible. To improve sensitivity, methods are here described to reduce the noise from technical background scattering.
In the highest-sensitivity system, the only source of noise would be the shot noise. However, there are other sources of noise (technical background scattering) that result in lowered sensitivity and therefore, result in an increase in the size of the smallest detectable particle. One of these additional sources of noise comes from laser amplitude fluctuations. These fluctuations appear as technical background scattering on the molecular-scattered light signal above the shot noise limit. One method used to reduce the noise from laser amplitude fluctuations is to monitor the laser output to determine the fluctuations in the laser amplitude and subtract these fluctuations from the scattered light signal (or in general, any other desired signal). This type of direct subtraction, however, does not work as well as expected in a multi-line laser system. In a multi-line laser, several lasing components at different wavelengths (“lines”) compete for the overall gain of the system. The relative circulating intensity of the lines can fluctuate appreciably. For example, it is possible for the overall intracavity power to remain constant (that is, the sum of the line strengths remains fixed) while the relative line strength changes. This becomes problematic when the laser output light undergoes spectral filtering, caused for example, by the use of an output optic with spectrally non-uniform transmission. If the spectral transmission of light at a monitor of laser amplitude fluctuations is different than the spectral transmission at a monitor of a desired signal, a subtraction of the laser amplitude fluctuations from the desired signal will lead to imperfect noise cancellation. This effect is not generally appreciated, yet is present in most multi-line laser systems, due to imperfect optical components.
Some methods described to reduce noise in a laser system have been described. U.S. Pat. No. 4,798,465 (Knollenberg, Jan. 17, 1989) and continuation-in-part U.S. Pat. No. 4,893,928 (Knollenberg, Jan. 16, 1990) describe a particle detection device having background noise reduction. The noise reduction is achieved by use of a plurality of linear detectors, where each detector senses a portion of the optical path. The signals from the detectors are parallel processed to reduce the effect of background molecular scattering. U.S. Pat. No. 6,061,132 (Girvin, May 9, 2000) describes a particle counter having a dual detector array, wherein a detector in one array is used for noise cancellation, a detector in the other array is used to detect the signal from a particle, and the signals are subtracted to reduce the noise. U.S. Pat. No. 5,467,189 (Kreikebaum, Nov. 14, 1995) describes a particle sensor which subtracts background scattering signals from particle signals. U.S. Pat. No. 5,121,988 (Biesener, Jun. 16, 1992) describes a particle detector having monitoring of laser output power and adjustment of the current supplied to the laser to compensate. U.S. Pat. No. 6,414,754 (Johnson, Jul. 2, 2002) describes use of an ionic coloring agent on portions of the instrument to absorb stray light.
None of the above methods describes a method to cancel the laser amplitude noise fluctuation component in a multi-line laser system. An improved method of canceling the laser amplitude noise fluctuation component from a desired signal in a multi-line laser system is needed.